I don't think they can be trained for both. It's one or the other. Dogs trained to detect drugs get all excited and even paw at the bag when they sniff something, which wouldn't be a good idea if a bag is carrying explosives (explosive-sniffing dogs are trained to sit when they suss something). You'll probably see more of this, since the administration's proposal to consign lithium-ion batts to the cargo hold went down in flames (so to speak). Dogs are just another means of screening for explosives.